Nuclear tools have been used for several decades to determine the density of earth rock formations surrounding a borehole. The nuclear density tools rely on the Compton scattering of gamma-rays in the formation for the density measurements. A conventional density tool consists of a source of gamma-rays (or X-rays), at least one gamma-ray detector and shielding between the detector and the source, so that only scattered gamma-rays are detected. During density logging, gamma-rays from the tool source travel through the borehole, into the earth formation. The gamma-rays will be scattered by the electrons in the formation or the borehole and some of them will be scattered back to the detector in the logging tool. Depending on the spacing between the source and detector, the count rate of detected gamma-rays will either increase with increasing formation density (scattering term dominant) or decrease with increasing formation density (attenuation effect predominant). At intermediate spacings, both attenuation and scattering terms influence the response.
In an ideal logging situation, the borehole would have a uniform and straight shape. This uniform borehole would enable a density tool containing a detector to be in close proximity with the formation surrounding the borehole and there would be minimal tool standoff. Under these conditions, one detector would be sufficient for a density measurement.
However, because boreholes normally do not have a uniform and straight shape, one major concern in density logging is the logging tool contact with the borehole wall. Density logging tools can be engineered either as pad tools or as mandrel tools. In a mandrel tool the source and detectors are in the body of the straight cylindrical tool. The long stiff length of such an arrangement renders it difficult for the tool to stay in close contact with a non-uniform borehole wall. In pad tools, the detectors and, in most cases, also the logging source are mounted in a short, articulated pad which can move with respect to the tool body. A strong eccentralizer arm pushes the pad against the borehole wall and allows much better contact because of the much smaller length of the device. All density logging tools will also encounter mudcake built up on the formation wall, which prevents good contact. The density measurement needs to be compensated for this kind of standoff as well. Because of the shortcomings of the mandrel tools, these tools are only used if a pad tool cannot be engineered because of size or cost constraints.
Most modern density tools use an articulated pad which houses the detectors and the gamma-ray source. A backup arm pushes the pad against the formation. The short length of such a pad and the large eccentralizing force exerted by the backup arm assure very good contact of the pad with the formation in most circumstances. However, for tools with a small diameter, the use of a pad type construction becomes difficult or impossible. In these cases, the detectors are placed inside the tool housing (mandrel tool). Eccentralization is provided by a bow-spring and/or a caliper device with a backup arm. However, the much longer stiff length of the tool leads to a poorer application of the tool to the borehole wall and leads to a larger average standoff.
The basic layout for a two detector tool is shown in FIG. 1. The tool 1 consists of a gamma-ray source 2, a short spaced (SS) detector 3 and a long spaced (LS) detector 4. The tool is in a borehole 5 that is substantially uniform. Gamma-rays emitted from the source 2 go into the borehole and earth formation 6, where they are scattered and some of them are subsequently detected by the detectors. The SS detector 3 is more sensitive to the region close to the tool 7. The LS detector 4 detects gamma-rays 8 from the formation 6 at greater depth than the SS detector and is less sensitive to effects of tool standoff. The apparent density derived from the LS detector measurement can be corrected for tool stand off by comparing the apparent density readings of the LS and SS detectors.
The correction for standoff caused by mudcake build-up or tool standoff can be accomplished by using two detectors with different depths of investigation. In this case, the first detector (SS) has a shallow depth of investigation and is more sensitive to the borehole fluid or mudcake between the tool and the formation. A second detector (LS) at a longer distance from the source is less sensitive to the borehole environment and is more sensitive to the formation. The difference between the two detector readings can be transformed into a correction for standoff and mudcake. However, at larger standoffs due to an irregular borehole shape 9 the 2-detector compensation is often insufficient or ambiguous.
The shortcomings of the 2-detector measurement lie in the fact that the two detector measurement is used to determine three unknowns: Formation density, standoff (distance between the tool and the borehole wall) and the density of the fluid and/or mudcake between the tool and the formation. At small standoffs the latter two unknowns can be combined into an effective thickness (mud density * standoff). At larger standoffs this approach fails and the correction becomes ambiguous. In addition, the short space detector depth of investigation can become smaller than the stand off. This will prevent proper compensation.
As shown in FIG. 1, the irregular shape of the borehole wall 9 causes the tool to be separated from the wall by a large distance. The short space detector 3 depth of investigation is smaller than the standoff and therefore an effective compensation of the density answer of the long space detector 4 is more difficult or nearly impossible to obtain.
The use of an additional detector positioned between the traditional LS and SS detectors can help in addressing the ambiguity of the correction at large tool stand off and some of the limitations of the two-detector tool can be overcome. The three-detector measurement provides the ability to distinguish the effect of the mud and/or mudcake thickness from the effect of the density of the mud/or and mudcake between the tool and the formation. In addition, the better statistical precision provided by the middle measurement will improve the logging speed of the tool. The operation of a three-detector tool is shown in FIG. 2. The three-detector tool 11 has the ability to measure three distinct depths of investigation in the formation. The tool has a source 12, and short spaced (SS) 13, middle spaced (MS) 14 and long spaced (LS) 15 detectors. Because of the shape of the borehole wall 9 a very large standoff 23 occurs between the tool 11 and the borehole wall 9. In order to compensate for the effect of this large standoff, at least two detectors must have depths of investigation greater than the tool standoff. Detectors 14 and 15 have depths of investigation, 25 and 26 respectively, that extend into the formation 6 and provide for the measuring of the formation and the material in the region 23 between the tool and the borehole wall.
The idea of using three detectors to differentiate different depths of investigation was described in U.S. Pat. No. 4,129,777 (Wahl). In Wahl, the main idea is to measure the density of material at three different depths from the tool. This can be used for determining formation density though casing, for determining the cement thickness behind casing or for determining mudcake density and thickness between the tool and the formation. In all three cases the measurement is also used to determine the formation density and the thickness and density of the a layer of material between the tool and the formation.
In Wahl, gamma radiation is emitted from the tool into the surrounding media and measurements are taken of the amount of radiation which returns to the detectors as a result of the interaction of the emitted radiation with first, second and third layers respectively of the surrounding media each beginning at the borehole and extending to increasing radial depths. These measurements are taken by three detectors located at different spacings from the gamma radiation source so as to have three different depths of investigation. A representation of the thickness of the solid matter is then obtained from the three gamma radiation measurements.
In particular, the method proposed by Wahl is useful for determining the thickness of the bonding material between a borehole casing and the adjacent formation. In that case, the three gamma radiation measurements (shallow, intermediate and deep) are corrected for the attenuating effect of the casing. Three densities are then computed from the shallow, intermediate and deep radiation measurements respectively.
Another patent incorporating the three detector concept is U.S. Pat. No. 5,525,797, Moake. In this patent, like in Wahl, the gamma-ray source is spaced axially from the first, second and third detectors. The first/near detector is axially spaced from the gamma source by a distance defined as a first spacing. The first spacing and collimation for the first detector are designed so that the gamma-rays detected at the first detector are those gamma-rays that are scattered primarily by the casing.
A second or middle detector is spaced axially farther away from gamma-ray source than the first detector. The second detector is spaced from the gamma-ray source by a distance defined as a second spacing. The second spacing and collimation for the second detector are designed so that the gamma-rays detected at the second detector will be those that are primarily scattered by the casing and the cement. Finally, a third or far detector is spaced axially farther away from the gamma-ray source than both the first and second detectors by a distance defined as a third spacing. The third spacing and collimation defined by the third detector are designed so that the gamma-rays detected at the third detector are those primarily scattered from the casing, cement and formation. It is this third detector that enables the tool to measure formation density while the first and second detectors primarily enable the tool to correct for casing and cement. However, the second detector can be used to measure formation density in the absence of cement.
Preferably, the detectors are shielded by a high density material between the source and the detector that prevents detection of gamma-rays that are simply traveling up through the tool. A pathway or void in the shielding is provided in the form of a collimation channel which extends from the detector through the tool and terminates at the outside surface of the tool. The collimation channels are specifically designed for the detection scheme of each detector. Specifically, the near or first detector will have a collimation that is claimed at a small angle with respect to the casing so that the first detector will detect gamma-rays that are scattered mainly by the casing. The second or middle detector will have a collimation that is directed at a steeper or more perpendicular angle with respect to the casing because the second detector is intended to detect gamma-rays scattered through all of the cement as well as the casing (deeper depth of investigation). Finally, the third or far detector will have a wide collimation channel which is directed substantially perpendicular to the casing due to the distance of the third detector from the source. Because gamma-rays detected at the far detector must pass through the casing, cement, formation before passing back through the cement and casing, the statistical probability of this event happening is smaller than for the first and second detectors and therefore a wider collimation channel is required for the third detector.
The three detector density presented by Wahl describes the general idea of using three detectors to measure density in the presence of a material of substantial thickness and/or density between the tool and the formation. The distinction between different depth of investigation is achieved by the different axial spacing of the detectors.
The invention presented by Moake uses substantially the same detector spacings as the invention of Wahl. The detector collimation is optimized for a through casing measurement. The SS (first) and LS(third) detectors use collimation which is very similar to the one used in traditional two-detector density tools. The MS (middle) detector collimation is very tight and almost perpendicular to the borehole wall to get a deeper density reading in through-casing measurements. The steep collimation angle of the MS detector reduces its count rate and statistical precision. In an open hole measurement the depth of investigation of the MS and LS detectors will become very similar and the sensitivity to mudcake, which has a much smaller density than the steel casing, is reduced.
There remains a need for a solution to determining a correction for standoff in logging tools that can overcome these limitations. One possible approach for a 3-detector algorithm is described in U.S. Pat. No. 5,390,115 (Case and Ellis).
The present invention provides a new multi-detector algorithm optimized for situations in which a density tool encounters substantial standoff from the formation. The method of this invention can be implemented in conjunction with the multi-detector tool described in pending U.S. patent application Ser. No. 08/800,976 (Atty. Docket No. 20.2658) now U.S. Pat. No. 5,841,135 which is incorporated by reference.
In addition to determining formation density, this invention can also measure the photo-electric factor (PEF) of the formation. This measurement relies on the absorption of low energy gamma-rays through the photoelectric effect in the formation. Since the photo-electric effect depends strongly on the atomic number of the formation elements, it provides an indication of the lithology of the formation. Because photoelectric absorption preferentially removes low energy gamma-rays, the tool housing needs to allow passage of low energy gamma-rays to detectors inside the housing. This objective can be accomplished through the use of a window of a material with a low atomic number (Z) in the housing or through the use of a low-Z housing material like titanium. Typical window materials are beryllium and titanium. Housing materials can be titanium or for lower pressure requirements graphite or high-strength carbon compounds.